1. Field of the Invention
The present invention relates to an ignition coil for generating a spark between electrodes of a spark plug of an internal combustion engine. The invention also relates to a method for manufacturing the ignition coil.
2. Description of Related Art
An ignition coil for use in an internal combustion engine includes a primary coil, a secondary coil, a center core, and an outer core. The primary coil is provided coaxially with the secondary coil to face with the second coil in a radial direction. The primary coil includes a primary spool and a primary winding, which is wound on the primary spool. The secondary coil includes a secondary spool and a secondary winding, which is wound on the second spool. The center core is provided at an inner peripheral side of the primary and secondary coils, and the outer core is provided at an outer peripheral side of the primary and secondary coils. Both the cores are made of magnetic material. In the above construction, a magnetic circuit is formed through the two cores.
FIGS. 9 and 10 show an ignition coil including a primary coil 92, which includes a primary winding 921 and a primary spool 93 of a conventional structure. The primary spool 93 has circumferential overhangs 933 formed on the outer periphery of its end 932 at the low-voltage end of the primary coil 92. The primary coil 92 includes an inner winding layer 921A and an outer winding layer 921B. The inner winding layer 921A is formed by winding the primary winding 921 around the primary spool 93 from the spool end 932 at the low-voltage coil end to the other spool end at the high-voltage end of the primary coil 92. The outer winding layer 921 B is formed by winding the primary winding 921 around the inner winding layer 921A from the spool end at the high-voltage coil end to the other spool end 932. A first winding end portion 922A of the primary winding 921 for a start of winding engages with one of the circumferential overhangs 933. A second winding end portion 922B of the primary winding 921 for an end of winding engages with another one of the circumferential overhangs 933.
The second winding end portion 922B of the primary winding 921 needs to come into engagement with another one of the circumferential overhangs 933 extending from the outer winding layer 921B, which is wound around the inner winding layer 921 A. This makes it difficult to engage the second winding end portion 922B stably with the circumferential overhang 933 and may loosen the engagement of the second winding end portion 922B.
JP-2005-252233A (corresponding to U.S. Pat. No. 7,129,812) discloses a primary winding and a primary spool, which has multiple circumferential ridges formed at an outer peripheral surface of an upper end, and the primary winding is passed through the space between two of the circumferential ridges. Here, the circumferential ridges are formed so that the primary spool can be fixed firmly in a casing. However, the primary spool is not so devised as to inhibit the second winding end portion of the primary winding from getting loose.